geological society of africa presidential review, no. 7 retroarc
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Journal of African Earth Sciences 38 (2004) 225–242
www.elsevier.com/locate/jafrearsci
Geological Society of Africa Presidential Review, No. 7
Retroarc foreland systems––evolution through time
Octavian Catuneanu *
Department of Earth and Atmospheric Sciences, University of Alberta, 1-26 Earth Sciences Building, Edmonton, Alberta, Canada T6G 2E3
Abstract
Retroarc foreland systems form through the flexural deflection of the lithosphere in response to a combination of supra- and
sublithospheric loads. Supracrustal loading by orogens leads to the partitioning of foreland systems into flexural provinces, i.e. the
foredeep, forebulge, and back-bulge. Renewed thrusting in the orogenic belt results in foredeep subsidence and forebulge uplift, and
the reverse occurs as orogenic load is removed by erosion or extension. This pattern of opposite vertical tectonics modifies the
relative amounts of available accommodation in the two flexural provinces, and may generate out of phase (reciprocal) proximal to
distal stratigraphies. Coupled with flexural tectonics, additional accommodation may be created or destroyed by the superimposed
effects of eustasy and dynamic (sublithospheric) loading. The latter mechanism operates at regional scales, and depends on the
dynamics and geometry of the subduction processes underneath the basin. The eustatic and tectonic controls on accommodation
may generate sequences and unconformities over a wide range of time scales, both > and <106 yr.
The interplay of base level changes and sediment supply controls the degree to which the available accommodation is consumed
by sedimentation. This defines the underfilled, filled, and overfilled stages in the evolution of a foreland system, in which deposi-
tional processes relate to sedimentation in deep marine, shallow marine, and fluvial environments respectively. Each stage results in
typical stratigraphic patterns in the rock record, reflecting the unique nature of flexural and longer-wavelength controls on
accommodation. Predictable shifts in the balance between flexural tectonics and dynamic loading allow subdivision of the first-order
foreland cycle into early and late phases of evolution dominated by flexural tectonics, and a middle phase dominated by system-wide
dynamic subsidence. The early phase dominated by flexural tectonics corresponds to an early underfilled foredeep and a forebulge
elevated above base level, whose erosion and rapid progradation results in the formation of the forebulge (basal) unconformity. The
middle phase dominated by dynamic subsidence corresponds to a stage of system-wide sedimentation, when the forebulge subsides
below the base level and the foredeep goes from a late underfilled to a filled state. The late stage dominated by flexural tectonics
corresponds to the first-order overfilled stage of foreland evolution, when fluvial sedimentation is out of phase across the flexural
hingeline of the foreland system.
� 2004 Elsevier Ltd. All rights reserved.
Keywords: Foreland systems; Orogenic loads; Dynamic subsidence; Flexural provinces; Underfilled forelands; Filled forelands; Overfilled forelands
* Tel.: +1-7
E-mail add
0899-5362/$ -
doi:10.1016/j.
Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226
2. Controls on accommodation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227
80-4
ress:
see f
jafrea
2.1. Basin-scale controls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227
2.2. Additional controls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229
3. Stratigraphy of retroarc foreland systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 230
3.1. Underfilled forelands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232
3.2. Filled forelands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234
3.3. Overfilled forelands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235
4. Discussion and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237
4.1. First-order foreland cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237
92-6569; fax: +1-780-492-7598.
octavian@ualberta.ca (O. Catuneanu).
ront matter � 2004 Elsevier Ltd. All rights reserved.
rsci.2004.01.004
Proarc foreland systForede(proar
foreland b
Forebulge(peripheral
bulge)
Pro-lithosphere
accommodation creby static and dynam
viscous mantle / ast
KEY:
1. Static loads: - sublithospheric: slab pu - supralithospheric: tecto2. Dynamic load (sublithos
LOAD TYPES:
Fig. 1. Proarc an
controls on accom
Foreland systems
under a combinati
details.
226 O. Catuneanu / Journal of African Earth Sciences 38 (2004) 225–242
4.2. Higher-frequency foreland cycles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 238
4.3. Migration of foreland systems through time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239
4.4. Phanerozoic vs. Precambrian foreland systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 240
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241
1. Introduction
Within the context of plate tectonics, foreland sys-
tems are associated with convergent plate margins,
where fold-thrust (orogenic) belts form along the edge of
the overriding continent (Fig. 1). These mountain ranges
represent supracrustal loads that press the lithospheredown on both sides of the convergent plate margin,
generating accommodation via flexural deflection, i.e.
foreland basins (Beaumont, 1981; Jordan, 1981). The
newly created depozones are referred to as proarc (or
simply pro-) foreland basins, where placed in front of
the orogenic belt––on the descending (pro-lithosphere)
plate, or as retroarc (retro-) foreland basins, where
placed behind the orogenic belt––on the overriding(retro-lithosphere) plate (Fig. 1). One important differ-
ence between the proarc and retroarc foreland settings is
that the retro-lithosphere is subject to the manifestation
of additional mechanisms that create accommodation,
related to sublithospheric forces. Sublithospheric
‘‘loading’’ of the overriding plate is primarily caused by
the drag force generated by viscous mantle corner flow
coupled to the subducting plate, especially where sub-duction is rapid and/or takes place at a shallow angle
beneath the retroarc foreland basin (Mitrovica et al.,
1989; Gurnis, 1992; Holt and Stern, 1994; Burgess et al.,
1997). This corner flow-driven ‘‘dynamic’’ loading gen-
erates accommodation at continental scales, with sub-
sidence rates decreasing exponentially with distance
Orogen(tectonic load)
em Retroarc foreland systemForedeep(retroarc
foreland basin)
epcasin)
Forebulge(peripheral
bulge)Back-bulge
basin CRATON
Dynamic loading(corner flow)
Subducting Lithospheric Slab
Retro-lithosphere
Sublithospheric static load (slab pull)
ated ic loads
enosphere
llnic load, sediment and waterpheric): viscous drag force of mantle corner flow
d retroarc foreland systems––tectonic setting and
modation (modified from Catuneanu et al., 1997a).
form by the flexural deflection of the lithosphere
on of supra- and sublithospheric loads. See text for
away from the orogen in a cratonward direction (Fig. 1).
In addition to dynamic subsidence, the gravitational pull
of the subducting slab also contributes to the sublitho-
spheric loading of the retro-lithosphere (Fig. 1).
Excepting for dynamic loading, all other types of
supra- and sublithospheric subsidence mechanisms re-
late to the gravitational pull of ‘‘static’’ loads repre-sented by the subducting slab, the orogen, or the
sediment–water mixture that fills the foreland accom-
modation created by lithospheric flexural deflection
(Fig. 1). The static tectonic load of the orogen and the
sublithospheric dynamic loading are most often invoked
as the primary subsidence mechanisms that control
accommodation and sedimentation patterns in retroarc
foreland settings (Beaumont et al., 1993; DeCelles andGiles, 1996; Pysklywec and Mitrovica, 1999; Catuneanu
et al., 1999a, 2002). The static load of the sediment–
water mixture is only of secondary importance in the
formation of foreland basins, because accommodation
by flexural deflection must be created first, before sedi-
ments can start to accumulate.
Even though only one of several subsidence mecha-
nisms, tectonic loading by orogens provides the definingfeature of foreland systems, i.e. their partitioning into
flexural provinces: foredeep (foreland basin), forebulge
(peripheral bulge) and back-bulge (Fig. 1). As a matter
of semantics, the foreland system refers to the sum of
three flexural provinces, whereas the foreland basin only
refers to the foredeep flexural province. Along the flex-
ural profile, the uplift of the forebulge is virtually syn-
chronous with the subsidence of the foredeep, and iscaused by the rapid lateral displacement of sublitho-
spheric viscous mantle material as a result of litho-
spheric downwarp beneath the orogen and the adjacent
foredeep (C. Beaumont, pers. com., 2002). The issue of
whether the forebulge region may or may not receive
and preserve sediments depends on the interplay of
flexural tectonics and other mechanisms that control
accommodation, and will be tackled in subsequent sec-tions of the paper.
It can be concluded that foreland systems form by the
flexural deflection of the lithosphere in response to a
combination of supra- and sublithospheric loads. The
magnitude of this deflection varies with the amount and
distribution of loads, as well as with the physical attri-
butes of the lithosphere that is subject to flexural
deformation. This paper summarizes the mechanisms
Forebulge(~ λ/2)
Back-bulge(~ λ/2)
Foredeep(~ λ/4)
Flexural tectonics: partitioning of the foreland system in response to orogenic loading.
Load
Craton
Retroarc foreland system
O. Catuneanu / Journal of African Earth Sciences 38 (2004) 225–242 227
that control the formation and evolution of retroarcforeland systems, as well as their diagnostic stratigraphic
signatures. Field examples are mainly provided by the
case studies of the Western Canada and southern Afri-
can Main Karoo basins, but other Precambrian and
Phanerozoic foreland systems are also discussed for
comparison.
positive accommodationmean dynamic deflectioncomposite lithospheric profile
Dynamic subsidence: long-wavelength lithospheric deflection in response to subduction processes.
Interplay of flexural tectonics and dynamic subsidence:
Load
Fig. 2. Interplay of flexural tectonics and dynamic subsidence in ret-
roarc foreland systems. These two basin-scale controls on accommo-
dation are independent of each other, being related to static and
dynamic loading respectively, and may record amplitude fluctuations
over different time-scales. The balance between their rates controls the
direction and magnitudes of base level changes in the different flexural
provinces of the foreland system––see Catuneanu et al. (1999a) for a
full discussion. k ¼ flexural wavelength.
2. Controls on accommodation
Accommodation in retroarc foreland systems is pri-
marily controlled by tectonic forces, as tectonism is
responsible for the formation of this basin type in the
first place. Subsidence in these settings, whether related
to static or dynamic loads, is always differential, with
rates generally increasing towards the associated oro-genic belt. This gives the basin fill an overall wedge-
shaped geometry, which in turn demonstrates the reality
of tectonic tilt and hence the dominance of the tectonic
control.
The factors that modify the amounts of accommo-
dation in the basin may be classified on the basis of their
regional significance. One set of ‘‘basin-scale controls’’
includes allogenic factors that act in a predictablemanner at the scale of the basin, and may be used to
model the overall evolution of the basin. Additional
controls may locally modify the amounts of accommo-
dation, and affect only specific areas within the basin.
2.1. Basin-scale controls
The most important basin-scale controls on accom-
modation include flexural tectonics related to tectonic
loads, dynamic subsidence related to subduction-in-
duced corner flows, and sea level changes. Eustatic
fluctuations are global in nature and may affect depo-sitional processes in all types of basins; flexural tectonics
characterizes both types of foreland basins, in proarc
and retroarc settings; dynamic subsidence is diagnostic
for retroarc foreland systems. The following discussion
focuses on the two main tectonic controls on accom-
modation in a retroarc foreland setting, namely flexural
tectonics and dynamic subsidence.
Fig. 2 illustrates the separate and combined effects offlexural tectonics and dynamic loading. Flexure of the
retro-lithosphere under orogenic loading results in the
partitioning of the foreland system into foredeep, fore-
bulge and back-bulge flexural provinces. This litho-
spheric deflection resembles the shape of a sine curve,
with the amplitude attenuated with distance. The
amounts of foredeep subsidence and forebulge uplift,
which define the vertical scale of the flexural profile, areproportional to the mass of the applied orogenic load,
and inversely proportional to the flexural rigidity of the
lithosphere. The attenuation with distance of the sinu-
soidal flexural profile is very rapid, such that the mag-
nitude of forebulge uplift is generally at least 20 times
less than the amount of foredeep subsidence (Crampton
and Allen, 1995). Accordingly, while the flexural
downwarp of the foredeep is usually in a range of
kilometers, the magnitude of forebulge uplift is generallyless than 200 m (Crampton and Allen, 1995; Catuneanu
and Sweet, 1999).
The wavelength of the sinusoidal flexural profile,
defining the horizontal scale of the foreland system,
varies with the basin, depending primarily on the rhe-
ology and thickness of the underlying lithosphere
(Watts, 1992; Beaumont et al., 1993). For comparison,
an effectively elastic plate (infinite relaxation time)would generate a wider foreland basin than a visco-
elastic plate (finite relaxation time), and so would a
thicker, older or less deformed plate (with higher flex-
ural rigidity and longer relaxation time). Computer
modeling of foreland systems shows that for an effec-
tively elastic lithosphere, the foredeep may reach a few
hundred kilometers in width, up to more than 400 km
for thicker plates (Johnson and Beaumont, 1995). Anexample of a foreland system developed on continental
lithosphere with high flexural rigidity is the Western
Canada Basin, where the hingeline separating the fore-
deep from the forebulge has been mapped as far as 350
km away from the orogenic front (Catuneanu et al.,
1997b). Similar distances describe the scale of the Karoo
foreland system (Main Karoo basin, southern Africa),
which also formed on thick and old (high flexuralrigidity) Precambrian crust, largely of the Kaapvaal
craton (Catuneanu et al., 1998). Where the foreland
228 O. Catuneanu / Journal of African Earth Sciences 38 (2004) 225–242
system develops on a less rigid and fractured lithosphere(modeled as visco-elastic), the foredeep may be much
narrower, such as in the case of the Alpine ‘‘molasse’’
basins with a width of less than 150 km (Homewood
et al., 1986; Crampton and Allen, 1995). Young and
therefore less rigid plates also generate narrow fore-
deeps, which is generally the norm with the older, Pre-
cambrian foreland systems that formed on newly
cratonized continental lithosphere. A good example isthe Late Archean Witwatersrand foredeep in South
Africa (Kaapvaal craton), with a width of only about
130 km (Catuneanu, 2001).
One important feature of the flexural profile of a
foreland system, irrespective of actual size, is the relative
proportion between the extent of flexural provinces, as
measured along dip (Fig. 2). The uplifted peripheral
bulge and the back-bulge depozone are both signifi-cantly wider than the foredeep, each measuring about
half of the wavelength of the sinusoidal flexural profile.
This may be explained by comparing the lithospheric
flexural profile with a transversal wave that is attenuated
with distance (looses energy) away from the source.
Along the direction of propagation of such a wave, each
positive or negative deflection corresponds to half of its
wavelength. The foredeep is also part of a half wave-length portion of the flexural ‘‘wave’’, but as the other
part of the same downwarp is occupied by orogenic
structures, the foredeep is invariably the narrowest
flexural province, extending only about a quarter of the
wavelength along dip (Fig. 2). Following Turcotte and
Schubert’s (1982) modeling of the flexural response of
an elastic plate floating above a fluid mantle substrate,
Crampton and Allen (1995) also reached the conclusionthat the wavelength of the deflection is a constant for the
entire flexural profile of the foreland system, assuming
that the plate is homogeneous.
According to Turcotte and Schubert’s (1982) theo-
retical considerations, expanded by DeCelles and Giles
(1996) to include the flexural subsidence of the back-
bulge basin, the wavelength of the flexural deflection of
an elastic plate under loading depends on the flexuralparameter of the lithosphere (a), and equals 2pa (for an
infinite plate) or 3pa=2 (for a broken plate). Based on
these formulae, the horizontal width of flexural prov-
inces measured along the dip of an infinite plate is pa=2for the foredeep, and pa for both the forebulge and the
back-bulge regions. These formulae may be extrapolated
to broken plates, by introducing a multiplication coef-
ficient of 3/4. The flexural parameter varies with therheology of the lithosphere and the contrast in density
between the mantle and the basin fill, and may range
from tens to hundreds of kilometers.
If flexural tectonism related to orogenic loading was
the only mechanism controlling accommodation in the
foreland system, the uplifted forebulge area would never
receive and preserve sediments. Under such circum-
stances, the peripheral bulge would be subject to erosionduring the entire evolution of the basin. This is the case
with a number of foreland systems, including the Alpine
molasse basins, or the Witwatersrand Basin. In such
cases, the depositional areas are generally restricted to
the foredeep and the back-bulge depozones (e.g., see top
cross-section in Fig. 2 for a conceptual illustration of
this principle; also, see Catuneanu, 2001, for a case
study). Other foreland systems however accumulatesediments across all flexural provinces, resulting in the
formation of foreland-fill wedges close to 1000 km wide.
What accounts for these differences?
Independent of flexural tectonics, dynamic subsi-
dence generates accommodation at continental scales,
with rates decreasing exponentially with distance from
the subduction zone (Fig. 2). This additional long-
wavelength lithospheric deflection may potentially bringthe entire foreland flexural profile below the base level,
as illustrated by the composite lithospheric profile in
Fig. 2. Under these circumstances, the forebulge region
may receive sediments for as long as the rates of dy-
namic subsidence exceed the rates of flexural uplift due
to orogenic loading. The balance between the rates of
dynamic loading and flexural tectonics is the key in
determining the direction and magnitude of base levelchanges in the different flexural provinces of the fore-
land system, and implicitly the stratigraphic architecture
across the foreland system (see Catuneanu et al., 1999a,
for a full discussion). For example, the dominance of
dynamic loading over the effects of tectonic loading
implies base level rise across the entire system, even
though with contrasting rates across the flexural hinge-
line that separates the foredeep from the forebulge, and,depending upon sediment supply as well, the basin may
likely be dominated by marine sedimentation. In con-
trast, a stage of flexural uplift outpacing the rates of
dynamic subsidence leads to the formation of unconfo-
rmities, commonly restricted to the flexural province
that is subject to uplift, and the basin tends to be
dominated by nonmarine sedimentation. The major
transgressive–regressive cycles observed in most fore-land systems may in part be related to this changing
balance between dynamic and static loading, and of
course also in part by fluctuations in sediment supply
(Catuneanu et al., 1999a).
The composite lithospheric profile in Fig. 2 (average
flexural profile in Fig. 3) changes through time in re-
sponse to fluctuations in the amount of orogenic load-
ing. Such orogenic cycles of thrusting (loading) andquiescence (erosional or extensional unloading) may
operate over a wide range of time scales, both > and <1
My (Cloetingh, 1988; Cloetingh et al., 1985, 1989; Peper
et al., 1992; Catuneanu et al., 1997b; Catuneanu and
Sweet, 1999). Renewed thrusting (loading) in the oro-
genic belt generates subsidence in the foredeep and uplift
of the forebulge, and the reverse occurs during orogenic
Forebulge Back-bulge
average accommodationaverage flexural profileflexural profile during loadingflexural profile during unloading
Foredeep Craton
Flexural hingeline
Fig. 3. Flexural response to orogenic loading and unloading (modified
from Catuneanu et al., 1997a). Renewed thrusting in the orogenic belt
(loading) results in foredeep subsidence and forebulge uplift. The re-
verse occurs during stages of orogenic quiescence (erosional or
extensional unloading): the foredeep undergoes uplift as a result of
isostatic rebound, compensated by subsidence of the forebulge.
OrogenicLoading
Hf
DATUM
sea-level
OR
OG
ENIC
FRO
NT
SEAWAY
FO
RE
BU
LG
E
increase
dlo
adin
g
CR
AT
ON
fore
deep a
xis
flexura
l hin
ge lin
e
back-b
ulg
e a
xis
(flexural hinge line)
Foredeep Forebulge Back-bulgebasin
Wedge-top
flexural subsidenceflexural uplift
0 300km
Fig. 4. Configuration of a foreland system during orogenic loading
with strike variability (modified from Catuneanu et al., 2000). The
topographic elevation of the adjacent craton, approximated with a
horizontal plane, is taken as a datum. The apex of the forebulge is
below the elevation of the adjacent craton because of the effect of
dynamic loading (not shown). The base level of deposition within the
foreland system (� sea level of the interior seaway) may be in any
position (below, above, or superimposed) relative to the datum, al-
though surface processes on the craton (sedimentation, erosion) tend
to adjust the datum to the base level. Note that the magnitude of
forebulge uplift increases with the amount of foredeep subsidence, and
implicitly with the amount of tectonic loading.
O. Catuneanu / Journal of African Earth Sciences 38 (2004) 225–242 229
unloading: i.e., isostatic rebound of the foredeep, com-
pensated by subsidence of the forebulge. Ongoing re-
search of Pleistocene and Holocene glacio-isostatic
cycles of crustal adjustment shows that the rise of the
forebulge is virtually synchronous with the subsidence ofthe foredeep, indicating rapid flow and pressure equili-
bration of the sublithospheric viscous mantle as a result
of changes in supracrustal loading (C. Beaumont, pers.
comm., 2002).
This flexural behavior of the foreland system in re-
sponse to orogenic cycles of loading and unloading
generates contrasting base level changes across the flex-
ural hinge line (Fig. 3). As inferred above, this contrastmay only be in terms of rates (high vs. low subsidence
rates, explaining, for example, the manifestation of
coeval transgressions and normal regressions in shallow
seas across the hingeline), or in terms of direction of base
level changes (rise vs. fall, explaining, for example, the
coeval formation of depositional sequences and strati-
graphic hiatuses across the hingeline). These contrasts in
the direction and/or the magnitude of base level shiftsacross the hingeline define the key diagnostic feature of
foreland systems, and are fundamental to understanding
the stratigraphic architecture of this basin type.
It is also important to note that the topographic
profile of foreland systems does not necessarily have to
parallel the shape of the lithospheric flexural profile. For
example, if the entire amount of accommodation shown
in Fig. 3 is consumed by sedimentation, the landscapetopography may be approximately flat, without showing
the presence of the flexural forebulge. From there, iso-
static rebound of the foredeep coupled with subsidence
of the forebulge (stage of orogenic quiescence) may lead
to a topography that is the mirror image of the litho-
spheric flexural profile. In this scenario, the foredeep
region of the foreland system may be topographically
more elevated than the subsiding forebulge region,which becomes a sag, even though the lithospheric
profile still has a flexural forebulge (Fig. 3). These as-
pects are tackled in more detail in the section that deals
with the stratigraphy of the retroarc foreland systems.
Figs. 2 and 3 are simple two-dimensional represen-
tations, which only explain the regional trends along
dip-oriented profiles. In a three-dimensional view how-
ever, the geology is complicated by the strike variability
in orogenic loading, which triggers an axial tilt in the
basin as illustrated in Fig. 4. This strike variability in
orogenic loading is generally the norm rather than theexception, as it is highly unlikely that a thrust sheet
would have exactly the same mass everywhere along the
strike of the fold-thrust belt. This differential loading
along strike generates a tilt in the direction of increased
loading, which in turn controls the direction of shoreline
transgressions and regressions, as well as the flow of
fluvial systems.
2.2. Additional controls
In addition to the basin-scale controls on the evolu-
tion of the foreland system, which provide the basis for
regional modeling (e.g., Figs. 1–4), the amounts ofavailable accommodation may be modified by the
manifestation of local factors, such as differential uplift
and subsidence of basement blocks (i.e., basement tec-
tonics), dissolution or displacement of salt deposits that
may be present in the subsurface, or differential com-
paction. These secondary controls on accommodation
generate stratigraphic ‘‘anomalies’’, i.e. departures from
the predicted geometry of sedimentary sequences that fillthe basin.
Basement tectonics, triggered by the reactivation of
crustal faults, is probably the most significant of these
1.25 λ ∼ 1,350 km
flexural profile
Foredeep
Foredeep
Forebulge
Forebulge
Back-bulge
Back-bulge
Bushveld BlockNamaqua--Natal Belt
Witwatersrand Block
colluvialalluvialfluvial
Continental ice sheetsFloating
ice
South North
Pietersburg Block
LimpopoBelt
ZimbabweCraton
Kaapvaal Craton
Offset dueto basementtectonics
Reference line(mean elevationof adjacent craton)
Fig. 5. Flexural and basement controls on the distribution of Late
Carboniferous Dwyka glacial facies in the Karoo Basin. The foredeep
subsided below the sea level, hosting sedimentation from floating ice
(dropstones within a marine silty matrix). The forebulge was elevated
above the sea level, supporting the formation of continental ice sheets.
The back-bulge area was fault-bounded to the south, hosting a half-
graben style of sedimentation with colluvial, alluvial, and fluvial facies.
The offset between the theoretical flexural profile and the actual field
cross-section is caused by the syn-depositional reactivation of the fault
boundary between the Bushveld and the Pietersburg basement blocks.
k ¼ flexural wavelength.
230 O. Catuneanu / Journal of African Earth Sciences 38 (2004) 225–242
secondary controls on accommodation. Differentialsubsidence and uplift of basement blocks may generate
mini-basins and arches (horst structures) within the
foreland system, with an apparent random distribution
that depends on the nature of the basement and the way
the intra-plate stress fields propagate and affect the
zones of weakness in the underlying crust. The
smooth and symmetrical sinusoidal flexural profiles
modeled in Figs. 2 and 3 are idealized, based on theassumption that the underlying basement is homoge-
neous. This is rarely the case in reality, as basements
tend to be heterogeneous, composed of blocks of dif-
ferent ages and lithologies, and hence with different
rheological properties.
A consequence of having a heterogeneous basement
underlying the foreland system is that the expected rel-
ative proportions along dip between flexural provincesmay be distorted. This is because the position of flexural
hingelines that separate the foredeep from the forebulge,
or the forebulge from the back-bulge basin, is poten-
tially controlled by the boundaries between basement
blocks with different rheologies, which may not neces-
sarily fit the inflexion points of the theoretical sine curve.
For example, the hingeline that outlines the Late
Campanian–Early Maastrichtian foredeep of the Wes-tern Canada foreland system is virtually superimposed,
in map view, on the limit between the Eyehill High and
the Medicine Hat Block/Vulcan Low Archean basement
provinces (Catuneanu et al., 1997b). Similarly, the hin-
geline between the foredeep and the forebulge of the
Karoo Basin follows closely the boundary between
the Namaqua-Natal Belt and the Kaapvaal Craton in
the underlying basement. In the same basin, the limitbetween the forebulge and the back-bulge depozone is
controlled by the contact between the Bushveld and the
Pietersburg blocks of the Kaapvaal Craton, which was
reactivated during the evolution of the basin (Catu-
neanu et al., 1999b). Fig. 5 illustrates the situation of the
Karoo Basin during the Late Carboniferous, where the
boundaries between flexural provinces, and implicitly
the distribution of syn-depositional glacial facies of theDwyka Group, are primarily controlled by basement
structures. In this example, the width of the forebulge is
greater than expected from the flexural modeling of a
homogeneous plate.
Besides the influence of basement tectonics and het-
erogeneity on the location of flexural hingelines, and
implicitly on the extent of flexural provinces, reactiva-
tion of crustal faults by foreland system tectonism mayalso control thickness and facies trends within individual
flexural provinces. As documented in a series of case
studies in the Western Interior foredeep of Canada and
the United States (e.g., Hart and Plint, 1993; Plint et al.,
1993; Pang and Nummedal, 1995; Donaldson et al.,
1998), flexure over reactivated crustal faults resulted in
varying rates of subsidence across basement block
boundaries, explaining localized incision, paleogeo-
graphic trends, and thickness and facies patterns that
parallel terrane boundaries in the basement. Theimportance of basement tectonics on sedimentation was
challenged by Aitken (1993), who noted that many
basement features are only occasionally reflected, if at
all, in the sedimentary cover overlying the basement.
The lack of correlation between some basement struc-
tures and the overlying sedimentary record may however
be caused by selective reactivation of crustal faults, and/
or by rapid ‘‘healing’’ of the underlying topography bythe oldest sedimentary sequences of the basin.
3. Stratigraphy of retroarc foreland systems
Retroarc foreland systems have a distinct strati-
graphic architecture relative to any other basin type,
which reflects the contrasts in the direction and/or themagnitude of base level shifts across flexural higelines.
The pattern of opposite flexural tectonics between the
foredeep and the forebulge (Fig. 3) modifies the relative
amounts of available accommodation in the two flexural
provinces, generating out of phase (reciprocal) proximal
to distal stratigraphies (Catuneanu et al., 1997a,b,
1999a, 2000; Catuneanu and Sweet, 1999).
Two styles of reciprocal stratigraphies have been de-fined in relation to the pattern of base level changes
across the foreland system (Catuneanu et al., 1999a).
One style refers to the case where the contrast in base
Prince Albert Formation
Collingham & Whitehill
Ripon Formation
Waterford and Fort BrownFormations
Ecc
a G
roup
fluvial marine glacial-marine
250
320
260
330
270
280
290
300
Ear
ly P
erm
ian
Ear
ly C
arbo
nife
rous
LateCarboniferous
LateCarboniferous
Lat
e P
erm
ian
Kungurian
Namurian
Serpukhovian
Kazanian
Sakmarian
Asselian
Artinskian
Visean
Tatarian
Ufimian
Foredeepstratigraphy
(M.y.)
Age
P
E
R
M
I
A
N
Dwyka Group
Filled phase(shallow marine)
Overfilled phase(nonmarine)
Und
erfi
lled
phas
e(d
eep
mar
ine)
Stages ofbasin evolution
Ear
lyL
ate
}Cape Supergroup
~ 30 My stratigraphic hiatus / forebulge (basal) unconformity
± onset of subduction and tectonic loading(end of extensional Cape Basin)
Beaufort Group
Fig. 6. Early Carboniferous to Late Permian evolution of southern
Africa, showing the change from extensional to compressional and
flexural tectonic regimes, as well as the stages in the evolution of the
Karoo foredeep.
O. Catuneanu / Journal of African Earth Sciences 38 (2004) 225–242 231
level changes is only in terms of rates (high vs. lowsubsidence rates across the hingeline), and consists of a
conformable succession of correlative transgressive and
normal regressive systems tracts. This case requires
continuous basin-wide sedimentation, with the rates
within the range of variation of the rates of base level
rise. A second style of reciprocal stratigraphies refers to
the case where base level changes across the flexural
hingeline take place in opposite directions, resulting insequences correlative to age-equivalent stratigraphic
hiatuses (sequence boundaries) in relation to coeval
rising and falling base level respectively. The shift from
one style to another in the evolution of a basin is linked
to the change in the balance between the rates of flexural
tectonics and the rates of longer-wavelength controls on
accommodation, such as dynamic subsidence and sea-
level changes (see Catuneanu et al., 1999a, for a fulldiscussion). Cyclic changes within the framework of this
balance may result in major (second-order) transgres-
sive–regressive shoreline shifts within the foreland sys-
tem, and implicitly in changes between dominant marine
or nonmarine sedimentation across the basin.
The interplay of base level changes and sediment
supply controls the degree to which the available
accommodation is consumed by sedimentation. Thisdefines the underfilled, filled and overfilled stages in the
evolution of the foreland system, in which deposi-
tional processes are dominated by deep marine, shallow
marine, or fluvial sedimentation, respectively (Sinclair
and Allen, 1992). The change from underfilled to over-
filled stages is best observed in the foredeep, because
the forebulge may be subject to erosion in the absence
of (sufficient) dynamic loading, or, at most, it mayaccommodate shallow marine to fluvial environments
even when the foredeep is underfilled. An example of
such a succession of stages is illustrated in Fig. 6, from
the case study of the Karoo Basin.
It can be noted that the evolution of any foreland
basin is somewhat predictable, starting with an under-
filled phase (‘‘flysch’’ style of sedimentation), and ending
with an overfilled phase (‘‘molasse’’ style of sedimenta-tion) (Crampton and Allen, 1995). Early forelands tend
to be underfilled because the onset of tectonic (orogenic)
loading is generally undercompensated by sediment
supply, due to the low elevation of the young orogenic
structures (Allen et al., 1986; Covey, 1986; Stockmal
et al., 1986; Desegaulx et al., 1991; Sinclair and Allen,
1992; Crampton and Allen, 1995). At the same time, the
forebulge associated with the earliest stage of forelandsystem evolution is always subject to erosion, and hence
recorded as an unconformity at the base of the foreland
basin-fill (basal/forebulge unconformity; Crampton and
Allen, 1995; Catuneanu, 2001; Fig. 6). This is because
the onset of dynamic loading lags in time behind the
initiation of subduction and tectonic loading, as it takes
time for the subducting slab to reach far enough beneath
the overriding plate to generate a viscous corner flow.
Once dynamic subsidence is high enough to outpace the
flexural uplift of the forebulge, the entire foreland sys-
tem may be lowered below the base level (Fig. 2). The
change in forebulge status from an erosional area to adepositional area may take place during the underfilled
stage of basin evolution, as in the case of the Karoo
Basin. The configuration of the earliest Karoo foreland
system accounts for a forebulge elevated above the base
level during the Dwyka time (Fig. 5; ‘‘early underfilled’’
stage in Fig. 6), followed by a time of system-wide
sedimentation during the Ecca time (‘‘late underfilled’’
stage in Fig. 6; Catuneanu et al., 1998, their Fig. 13).This transition may be explained by the initiation of
dynamic loading associated with the subduction of the
paleo-Pacific plate beneath Gondwana. In this example,
the lag time between the onset of subduction and tec-
tonic loading in the Namurian (Smellie, 1981; Johnson,
1991; Mpodozis and Kay, 1992; Visser, 1992), and the
onset of dynamic loading at the beginning of the
Permian was at least 40 My, corresponding more orless with the entire duration of the Late Carboniferous
(Fig. 6).
232 O. Catuneanu / Journal of African Earth Sciences 38 (2004) 225–242
Overfilled phases of dominantly fluvial sedimentationmark the late stages of foreland system evolution, when
sediment supply is high and both orogenic and dynamic
loading subside towards the end of the compressional
regime in the adjacent fold-thrust belt. Fig. 7 summa-
rizes the overall evolution of a retroarc foreland system,
from underfilled to overfilled. The initial underfilled
nature of the foredeep is generally the norm, as the
earliest tectonic loads are likely placed below the sealevel (not shown in Fig. 7; Stockmal et al., 1986; De-
segaulx et al., 1991; Sinclair and Allen, 1992) and hence
subject to little erosion. As sediment supply increases
through time with the gradual uplift of the fold-thrust
belt, sedimentation catches up with the available
1. Underfilled phase: deep marine environment in the foredeep
2. Filled phase: shallow marine environment across the foreland system
3. Overfilled phase: fluvial environment across the foreland system
Sea level
Sea level
Sea level
Forebulge Back-bulgeForedeep
Load(+, -)
Load(+, -)
Load(+, -)
Lithospheric flexural profile
Lithospheric flexural profile
Lithospheric flexural profile
Lithospheric flexural profile
Lithospheric flexural profile
Flexural uplift > dynamic subsidence
Flexural uplift > dynamic subsidence
Dynamic subsidence > flexural uplift
Dynamic subsidence > flexural uplift
Foreslope
Foredeep
Foresag
Forebulge
Fluvial depositionFluvial bypass/erosion
Subaerial erosionFluvial deposition
Foreland fill deposits
Differential isostatic rebound steepens the topographic foreslope
Differential flexural subsidence reduces the topographic gradient
Flexural subsidence
Flexural uplift
OROGENIC UNLOADING
OROGENIC LOADING
Load(-)
Load(+)
Fig. 7. Underfilled, filled and overfilled stages of foreland system
evolution. The fill of the foreland system corresponds to a first-order
cycle of changing balance between flexural tectonics and dynamic
subsidence. Note the difference between bathymetric/topographic
profiles and the lithospheric flexural profile. (+, )) refer to increases
and decreases in orogenic load, respectively.
accommodation and the deep seas become shallow seas,which eventually regress to make room for a dominantly
continental environment.
This first-order cycle of foreland evolution and sedi-
mentation correlates with a cycle of overall increase and
decline in the rates of dynamic subsidence (e.g., Py-
sklywec and Mitrovica, 1999), as the rates of subduction
increase from zero at the onset of compression, and
decrease back to zero towards the transition fromcompression to extension. It is thus expected that flex-
ural uplift outpaces dynamic subsidence in the earliest
and latest stages of basin evolution, with the dynamic
loading dominating the intermediate stage of basin-wide
shallow marine sedimentation (Fig. 7). This first-order
scenario only describes general trends, and second-order
(and superimposed on these, higher frequency) fluctua-
tions in the balance between the rates of flexural tec-tonics, dynamic subsidence, sea level shifts and
sedimentation are common and explain the complexity
of changes in depositional systems across the foreland
system with time. Forward modeling of such changes
has been used to simulate synthetic stratigraphic models
of foreland system development under specific condi-
tions of sediment supply and accommodation (e.g.,
Flemings and Jordan, 1989; Jordan and Flemings, 1991;Sinclair et al., 1991). The sections below provide
examples of underfilled, filled and overfilled stratal
stacking patterns from the case studies of the Karoo and
Western Canada foreland systems (Figs. 8 and 9).
3.1. Underfilled forelands
Underfilled foreland systems are defined by rapidly
increasing water depths in the foredeep, as accommo-
0 1500km
SUBDUCTION AND ACCRETION
KarooParana
BeaconBowen
Pan Gondwanian foreland system
thrust-fold belt
CFB
Fig. 8. Paleogeographic reconstruction of Gondwana, showing the
subduction from south to north of the paleo-Pacific plate beneath the
supercontinent. As a result of compression and terrain accretion, a ca.
6000 km long fold-thrust belt formed along the southern margin of
Gondwana, with an associated retroarc foreland system to the north.
Following the breakup of Gondwana, portions of this foreland system
are now preserved in South America, South Africa, Antarctica and
Australia. CFB¼Cape Fold Belt.
0 1500
Western Interior foreland
thrust-fold belt
km
SUB
DU
CT
ION
AN
DA
CC
RE
TIO
N
Western
Canada
forelandsystem
U.S.A.
Fig. 9. The Western Interior foreland system, in relation to the asso-
ciated fold-thrust belt and the convergent margin of North America.
Load(+/-)
composite lithospheric deflection (basement profile)
Ecca seaway
Not to scale.
Cap
eFold
Bel
t
transgres
sive shorel
ine
trans
gres
sive sh
oreli
ne
pelag
icfac
ies flooding
underfilled foreland system deposits
marine and fluvial aggradation
Forebulge Back-bulge
Fig. 10. Late underfilled stage in the evolution of a foreland system,
where increased dynamic subsidence results in the lowering of the
peripheral bulge below the sea level (modified from Catuneanu et al.,
2002). Transgressions of both proximal and distal shorelines of the
interior seaway may be recorded as accommodation outpaces the low
rates of sediment supply.
composite lithospheric deflection (basement profile)
(flexural subsidence, FOREBULGE)
(flexural uplift, FOREDEEP)
Ecca seaway
de frlta ontS. fa
n delta plain - fluvial aggradation
Not to scale.
Cap
eFold
Bel
t
forced
regres
sion
normalregression
Load(-)
underfilled foreland system deposits
Fig. 11. Late underfilled stage in the evolution of a foreland system,
where temporary dominance of flexural tectonics may result in the
manifestation of coeval forced and normal regressions of the proximal
and distal shorelines of the interior seaway (modified from Catuneanu
et al., 2002). In this example, submarine fans in the foredeep (‘‘S. fan’’
in the diagram) and fluvial-deltaic sequences over the forebulge pro-
grade at the same time into the interior seaway.
O. Catuneanu / Journal of African Earth Sciences 38 (2004) 225–242 233
dation is created faster relative to the rates of sedimen-
tation. This results in a relatively deep water environ-
ment, with water depths in a range of a few hundred
meters. Underfilled conditions characterize the early
stages of basin development, when tectonic loads are
below sea level or have a low elevation above sea level.
The initial subsidence of the foredeep is accompanied by
the flexural uplift of the forebulge above the base level,which leads to the formation of the basal (forebulge)
unconformity. This unconformity has the significance of
a first-order sequence boundary, as it separates the
sedimentary fill of an extensional basin, below, from
foreland basin deposits above (Fig. 6). Gradual in-
creases in the rates of dynamic loading with time may
lead to the lowering of the forebulge below the base
level, and hence to basin-wide sedimentation. These twostages are illustrated in the top two diagrams of Fig. 7.
This predictable trend of evolution allows us to separate
an early underfilled stage characterized by foredeep
sedimentation and forebulge emergence (flexural up-
lift > dynamic subsidence), followed by a late underfilled
stage of basin-wide sedimentation (dynamic subsi-
dence >flexural uplift) (Fig. 7).
The early underfilled stage is exemplified by the casestudy of the Dwyka Group in the Karoo Basin (Fig. 5).
The onset of emplacement of tectonic loads in the
Carboniferous led to subsidence and the establishment
of a glacial-marine environment in the foredeep,
accompanied by the elevation of a peripheral bulge
above sea level. The latter region, with a wavelength
controlled in part by basement heterogeneities (see dis-
cussion above), allowed for the formation of continentalice sheets (Visser, 1991).
The late underfilled stage applies to the underfilled
portion of the Ecca Group (Prince Albert to Ripon
Formations; Fig. 6), when sedimentation extended
across the entire foreland system, and the foredeep
accumulated pelagic to gravity flow sediments. At a
first-order level, dynamic subsidence outpaced the rates
of flexural uplift, leading to the lowering of the periph-eral bulge below the sea level and the manifestation of
basin-wide transgressions of the interior seaway (Fig.
10). At a higher frequency level, fluctuations in sediment
supply and in the balance between the rates of flexural
tectonics and dynamic loading may result in forced
regressions on the side of the basin that is subject to
flexural uplift, coeval with transgressions or normal
regressions of the opposite shoreline of the interiorseaway (Fig. 11). The latter is the case with the age-
equivalent Ripon and Vryheid formations in the Karoo
Basin, when submarine fans in the foredeep formed at
the same time as the progradation of fluvial–deltaic se-
quences over the forebulge (Catuneanu et al., 2002).
234 O. Catuneanu / Journal of African Earth Sciences 38 (2004) 225–242
This has significant implications for the petroleumexploration of underfilled foreland sequences, as both
the proximal turbidites and the distal deltas have the
potential to form good hydrocarbon reservoirs.
3.2. Filled forelands
The gradual increase in the amount of sediment
supply through time in response to tectonic uplift in thefold-thrust belt leads to a shallowing of the earlier un-
derfilled interior seaway and the establishment of a
shallow marine environment across the foreland system.
This defines the filled stage of foreland system evolution,
where accommodation and sedimentation are more or
less in balance (Fig. 7). Maintaining a system-wide
shallow marine environment requires that a number of
conditions are fulfilled, including (1) a forebulge loweredbelow sea level, hence (2) dynamic subsidence (or sea
level rise) outpacing the rates of flexural uplift, which in
turn implies (3) base level rise across the entire foreland
system, and (4) sedimentation rates that are within the
range of variation of the rates of base level rise.
The rates of dynamic subsidence are expected to peak
during this intermediate stage of foreland system evo-
lution, as discussed above, and hence the overall rates ofbase level rise are potentially at a maximum during filled
stages. This means that sedimentation rates also reach a
peak during these stages, and therefore the basin is most
active from both tectonic and depositional points of
view.
A case study for a filled foreland system succession is
provided by the Bearpaw Formation of the Western
Canada Sedimentary Basin (Fig. 12). This shallowmarine succession corresponds to the last basin-wide
incursion of the Western Interior seaway, which flooded
the entire foreland system from Manitoba to the Foot-
hills of Alberta. The overall (second-order) transgres-
B
Bp
Bp
BpU.S.A.
Saskatoon
CANADA
Drumheller
Calgary
Lethbridge
Red Deer
AL
BE
RTA
Edge
ofthrust-fold
belt
Post-BeaBearpawpre-Bearpforedeep
SASK
AT
CH
EW
AN
Edmonton
A
A’
Fig. 12. Geological map showing the location of the Bearpaw Formation in A
Cross-section A–A0 is shown in Fig. 13.
sion took place over ca. 3 My (Baculites scotti toBaculites compressus ammonite zones; Obradovich,
1993), whereas the overall regression lasted for about 9
My (Baculites compressus till the end of the Cretaceous;
Catuneanu et al., 2000). Sedimentation during this ca.
12 My second-order transgressive-regressive cycle was
generally continuous, as the succession is relatively
conformable. Several third-order transgressive–regres-
sive sequences have been mapped within the BearpawFormation, each corresponding to a flexural cycle of
orogenic loading and unloading (Catuneanu et al.,
1997b, 2000). The stratigraphic architecture shows coe-
val accumulation of transgressive facies in the foredeep
and normal regressive facies in the forebulge area during
stages of orogenic loading, and the converse situation
for stages of orogenic unloading (Fig. 13). This defines a
style of reciprocal stratigraphies that is typical for filledforelands (Catuneanu et al., 1999a).
A full discussion of the balance between the rates of
flexural tectonics, dynamic subsidence and sedimenta-
tion that allows for the formation of this style of re-
ciprocal stratigraphies is provided by Catuneanu et al.
(1999a). The base level rises across the entire foreland
system as dynamic subsidence outpaces the rates of
flexural uplift, but with higher rates in the region that issubject to flexural subsidence and with lower rates in the
region that is subject to flexural uplift. Given a sediment
supply that is within the range of variation of the rates
of base level rise, the higher subsidence areas are asso-
ciated with transgressions, and the lower subsidence
areas are associated with normal regressions. If this
balance is altered in the favor of sedimentation, i.e.
sedimentation> higher subsidence rates, then a basin-wide normal regression takes place, and a transition is
made towards an overfilled foreland system. This is the
case with the final regression of the Bearpaw seaway
(Fig. 13), which may be attributed to a decrease in the
Bp
p
P
MA
NIT
OB
A
Regina
SASK
.
0 15075
rpaw - mainly fluvial(Bp)/Pierre (P) - marineaw - mainly fluvialoutline (hingeline)
Studyarea(Km)
lberta and Saskatchewan (modified from Catuneanu and Sweet, 1999).
Belly River Formation
Hinge zone
25m
10 km
Foredeep (SW) Forebulge (NE)
Horseshoe CanyonFormation
Bearpaw
Formation
Ammonitezonation
Ammonitezonation
Baculitesjenseni
Baculitesjenseni
Baculitesreesidei
Baculitesreesidei
Baculitescuneatus
Baculitescuneatus
Baculitescompressus
Baculitescompressus
A A A’
transgressive faciesnormal regressive facieszone of facies transitionbentonite layers
marine to nonmarine facies contactmaximum regressive surfacemaximum flooding surfacewave ravinement surface
Fig. 13. Stratigraphic cross-section of correlation of gamma ray logs (A–A0 in Fig. 12), showing the distribution of transgressive and normal
regressive facies of the Bearpaw Formation in a time framework (modified from Catuneanu et al., 1997b). The underlying and overlying litho-
stratigraphic formations (Belly River and Horseshoe Canyon) are mainly composed of fluvial facies. The marine succession is relatively conformable
and shows a style of reciprocal stratigraphies typical for filled foreland systems, controlled by flexural cycles of orogenic loading and unloading.
CENTRAL ALBERTA SASKATCHEWAN MANITOBA/NORTH DAKOTA
Foredeep Forebulge
O. Catuneanu / Journal of African Earth Sciences 38 (2004) 225–242 235
rates of dynamic subsidence. If the lower subsidence
rates outpace the sedimentation rates, then a basin-wide
transgression takes place. A variety of situations cantherefore be envisaged and modeled, explaining the
variability observed in case studies. As a general trend,
the long-wavelength controls on base level rise (dynamic
subsidence and/or sea level rise) are dominant at the
onset of filled foreland stages, explaining the basin-wide
transgressions of the shallow interior seaways, and
subside toward the end of the filled stages, allowing the
transition to overfilled forelands.
Orogenupper Scollard Fm.
lower Scollard
Obed Marsh
Ardley zone/Ferris-Anxiety Butte
Whitemud Fm.
Whitemud Fm. Boissevain
Fm.
Carbon-Thompson
Hor
sesh
oeC
anyo
nFo
rmat
ion
Bea
rpaw
cycl
eC
anno
nbal
lcy
cle
Pask
apoo
Fm.
Coulter Mbr.
Odanah Mbr.Eastend Fm.
Pierre Shale
#0- 9
coal
s
Coal #10
Frenchman Fm.
Turtle Mtn. Fm.
Ravenscrag Fm.
Boundary - Estevan
"Willowbunch"
Belly River (Judith River) Group
Bearpaw Fm.
Not to scale
hingeline coal zone
unconformity unconformity-correlative strata
correlation line
marine and marginal marine facies nonmarine facies
Battle Fm.
Cam
pani
anM
aast
rich
tian
Pale
ocen
e
Fig. 14. Generalized dip-oriented cross-section through the Western
Canada foreland system, showing the overfilled style of reciprocal
stratigraphies in the post-Bearpaw fluvial succession (modified from
Catuneanu et al., 1999a). Note that each major unconformity is re-
stricted to the one side of the basin that was subject to flexural uplift at
the time.
3.3. Overfilled forelands
Overfilled foreland systems are dominated by non-
marine environments, and reflect stages in the evolution
of the basin when sediment supply outpaces the avail-
able accommodation (Fig. 7). At a first-order level, the
overfilled stage represents the final phase in the evolu-
tion of a foreland system, when the rates of dynamicsubsidence decrease below the rates of flexural uplift.
This shift in the balance between the two main controls
on foreland accommodation results in a half-system
style of sedimentation, where only one flexural province
(the one subject to flexural subsidence) receives sedi-
ments at any given time. At higher frequency levels,
foreland systems may reach an overfilled state any time
sedimentation exceeds the rates of base level rise, whichmay happen during the dominance of either flexural
tectonics or dynamic subsidence. In the latter case, ba-
sin-wide nonmarine sequences may be generated.
An example of typical overfilled regional architecture
is illustrated in Fig. 14, from the case study of the
Western Canada foreland system. In contrast to theunderlying marine Bearpaw Formation, which is rela-
tively conformable and displays a filled style of re-
ciprocal stratigraphies, the post-Bearpaw nonmarine
South (proximal) North
A
B
C
D
E
F
1
2
3
45
6
7
200 m 10 km
sand-bed braided systemssand-bed meandering systemsfine-grained meandering systems
Vertical profile: fining-upward at the level of an individualsequence, and coarsening-upward at larger scale
Fig. 15. Internal architecture of the Balfour Formation along a dip
oriented profile through the overfilled Karoo foredeep (modified from
Catuneanu and Elango, 2001). Changes in fluvial style within each
sequence generate fining-upward trends, as a response to decreasing
slope gradients through time. At a larger scale, the overall vertical
profile is coarsening-upward due to the gradual progradation of the
orogenic front. A–F¼ third-order fluvial sequences; 1 and 7¼ second-
order sequence boundaries; 2–6¼ third-order sequence boundaries.
236 O. Catuneanu / Journal of African Earth Sciences 38 (2004) 225–242
succession is marked by the presence of significant un-conformities. These unconformities are associated with
stratigraphic hiatuses of at least 1 My, and are restricted
to the flexural province that was subject to uplift at the
time of unconformity formation. This demonstrates the
fact that flexural tectonics outpaced the rates of dynamic
subsidence, and defines the overfilled style of reciprocal
stratigraphies in which each unconformity has an age-
equivalent depositional sequence on the opposite side ofthe flexural hingeline (Fig. 14). Flexural tectonism is
therefore the dominant control on the overfilled fore-
land stratigraphy––stages of orogenic loading result in
foredeep sequences and coeval forebulge unconformities
(sequence boundaries), whereas stages of orogenic
unloading lead to the formation of sequence boundaries
in the foredeep and age-equivalent sequences in the
forebulge area (Figs. 7, 14). Even though of secondaryimportance in terms of rates, dynamic subsidence is still
active during this overfilled stage, as evidenced by the
preservation of proximal and distal depositional se-
quences. In the absence of dynamic loading, each newly
accumulated foredeep or forebulge sequence would be
eroded during subsequent flexural uplift.
The vertical profiles of foredeep and forebulge fluvial
sequences are primarily a function of the changesthrough time in the type of rivers that bring sediment
from the source areas. In turn, the style of fluvial sys-
tems is controlled by changes through time in topo-
graphic gradients. The reason these gradients change is
because both flexural subsidence and isostatic rebound/
uplift during stages of orogenic loading and unloading
take place with differential rates. For example, the
topographic slope of the foredeep area (‘‘foreslope’’ inFig. 7) becomes increasingly steeper during orogenic
unloading, as the rates of isostatic rebound are highest
in the fold-thrust belt and gradually decrease towards
the subsiding forebulge. During such a stage, the
foreslope is subject to bypass and/or erosion and the
subsiding forebulge area (‘‘foresag’’ in Fig. 7) receives
coarser and coarser sediments brought by rivers that are
characterized by increasingly higher energy levels(Catuneanu and Sweet, 1999). During orogenic loading,
differential flexural subsidence in the foredeep, with the
highest rates in the center of loading, reduces the
foreslope gradient, which lowers the energy level of
fluvial systems through time (Fig. 7). This leads to the
accumulation of fining-upward foredeep fluvial se-
quences, as shown in Fig. 15. Note that the fining-up-
ward trend characterizes each individual sequence, indirect response to lowering slope gradients, but the
overall vertical profile of the overfilled foredeep may be
coarsening-upward due to the progradation through
time of the orogenic front (Fig. 15; Catuneanu and
Elango, 2001).
In addition to the differential subsidence along dip,
subsidence may also be differential along strike due to
the variability in orogenic loading. This leads to changes
in syn-depositional tilt, hence to changes in paleo-
flow directions both within a sequence (Catuneanu
et al., 2003) and across sequence boundaries (Catuneanu
and Elango, 2001). In spite of the differential rates of
subsidence and uplift that characterize the evolution ofany foredeep basin, sequence boundaries observed at
outcrop scale do not necessarily show angular relation-
ships because the slope gradients may only vary within a
±2% range, which is however enough to generate
changes in fluvial styles during the deposition of each
sequence.
As the duration of loading stages in the fold-thrust
belt is generally much shorter in time relative to theduration of quiescence stages, the overfilled foredeep
stratigraphy only preserves a fraction of the geological
record, with most of the time being absorbed within
sequence boundaries. A generalized model of out-of-
phase foreland sedimentation during an overfilled stage,
showing vertical trends of foredeep and forebulge fluvial
sequences, is illustrated in Fig. 16.
The seesaw patterns of sedimentation described inthis section are similar to the ‘‘antitectonic’’ model for
foreland system development proposed by Heller et al.
(1988). This model also predicts the most active proxi-
mal sedimentation during thrust loading, followed by
deposition in the forebulge area during post-orogenic
tectonic rebound, when the foredeep is subject to ero-
sion. The topographic profile of the overfilled foreland
system changes significantly during these loading andunloading stages (Fig. 7), causing predictable shifts in
the types of fluvial drainage systems (i.e., axial vs.
transversal). Orogenic loading induces a proximal axial
Fig. 16. Generalized model of fluvial sedimentation in overfilled
foreland systems dominated by flexural tectonism (modified from
Catuneanu and Sweet, 1999). Stages of orogenic loading are shorter in
time relative to stages of orogenic unloading. The former result in the
accumulation of fining-upward foredeep sequences, the latter lead to
the deposition of coarsening-upward forebulge (topographic foresag)
successions. Sedimentation across the flexural hingeline is out-of-
phase, which defines the overfilled style of reciprocal stratigraphies.
Note that the vertical axis indicates time––due to the increase in sub-
sidence rates in a proximal direction, the foredeep sequences are
actually thicker than the forebulge ones. Abbreviations: U¼uplift;
S¼ subsidence; N/E¼ non-deposition or erosion; AF¼ alluvial fans;
B¼braided fluvial systems; M¼meandering fluvial systems;
La¼ lacustrine systems; C¼ coal seams; P¼ paleosols; IV¼ incised
valleys.
O. Catuneanu / Journal of African Earth Sciences 38 (2004) 225–242 237
(longitudinal) drainage system which is channelizedalong the subsiding foredeep that appears ‘‘underfilled’’
relative to the uplifted orogen and forebulge that flank it
on both sides (Jordan, 1995). The actual direction of
flow depends on the direction of tilt in the basin, which
in turn is controlled by the strike variability in orogenic
loading. Such axial drainage systems follow the axis of
the foredeep, and receive tributaries from both the
orogen and the forebulge (Jordan, 1995). Isostatic re-bound during orogenic unloading modifies the topo-
graphic profile in such a way that the dominant drainage
system becomes transversal across the proximal fores-
lope (Jordan, 1995). In this case, the dominant flow may
only become longitudinal along the axis of the topo-
graphic foresag (Fig. 7). Such a change from a proximal
to a distal axial flow has been documented for the Pli-
ocene section of the Himalayan foreland system of In-dia, based on the shifts in the position of the Ganges
river system (Burbank, 1992).
4. Discussion and conclusions
4.1. First-order foreland cycle
The life span of a retroarc foreland system starts and
ends with events of inversion tectonics at the associated
continental margin, from extension to compression and
from compression to extension respectively. For exam-
ple, in the case of the Karoo, the change from a diver-gent continental margin to a convergent plate margin
took place towards the end of the Early Carboniferous
(Smellie, 1981; Johnson, 1991; Mpodozis and Kay, 1992;
Visser, 1992), and the end of the compression and hence
foreland system evolution was marked by the initiation
of the Gondwana breakup in the Middle Jurassic (ca.
183 My ago; Duncan et al., 1997). The first-order fore-
land cycle thus lasted for about 117 My (300–183 My).As extension on one side of the globe needs to be
compensated by compression elsewhere, the subduction
and accretion of terrains leading to the initial defor-
mation of the Western Canadian Cordillera started in
the early Middle Jurassic (ca. 180 My ago) and lasted
until the onset of extension in the Early Eocene (ca. 55
My ago; Monger, 1989). The Western Canada foreland
system cycle thus lasted for about 125 My (180–55 My).Accommodation during such first-order foreland
cycles is primarily controlled by the interplay of orogen-
driven flexural tectonics and subduction-induced
dynamic subsidence. The rates of flexural uplift of the
peripheral bulge during stages of orogenic loading may
be considered as approximately constant during the
evolution of the basin, assuming that each thrust sheet
brings about the same mass of supracrustal load. Incontrast, the rates of dynamic subsidence increase fol-
lowing the onset of subduction, and decrease towards
the end of the compressional cycle (Fig. 17). This allows
the inference of a general trend in the evolution of
accommodation across the foreland system, with (1)
early dominance of flexural tectonics (underfilled fore-
deep with the formation of the forebulge unconformity);
(2) mid-cycle dominance of dynamic subsidence (system-wide sedimentation, with the forebulge lowered below
base level: late underfilled and filled stages); and (3) late
dominance of flexural tectonics (overfilled stage, with
sedimentation restricted to one flexural province at a
time) (Fig. 17). Cessation of dynamic loading following
the end of the foreland cycle leads to continental scale
uplift (Fig. 17; e.g., Pysklywec and Mitrovica, 1999).
While the rates of flexural uplift may be consideredconstant during the first-order foreland cycle (Fig. 17),
the periodicity of orogenic pulses changes through time
following the same pattern as the curve that describes
the change in the rates of dynamic subsidence. In other
words, flexural cycles are longer in time in the early and
late stages of foreland basin evolution (less frequent
terrain accretion events at the convergent margin in
Onset of subductionand tectonic loading
End of subductionand tectonic loading}
Foreland cycle
Rat
es
Time
flexural upliftof forebulge
Under-filled
Over-filledFilled
dynamic subsidenceof forebulge
dynamicuplift
(1) (2) (3)
{ {{
early late
Fig. 17. Relative balance between the rates of flexural uplift and dy-
namic subsidence during a first-order foreland cycle, based on the case
studies of the Karoo and Western Canada foreland systems. While the
rates of flexural uplift may be considered constant during the first-
order foreland cycle (i.e., each thrusting event brings approximately
the same mass of supracrustal load), the periodicity of orogenic pulses
changes through time following the same pattern as the curve that
describes the change in the rates of dynamic subsidence––see text for
more details. The basal (forebulge) unconformity forms during stage
(1) of early underfilled state, when the rates of flexural uplift outpace
the rates of dynamic loading. Stage (2) reflects an overall dominance of
dynamic loading over flexural tectonism, whereas stage (3) corre-
sponds to the decline in dynamic loading, when the system is again
dominated by flexural tectonism. Stage (2) tends to start with a late
underfilled state, followed by the filled phase of equilibrium between
the rates of sedimentation and base level rise. The cessation of dynamic
loading at the end of the foreland cycle causes long-wavelength
lithospheric rebound, i.e. dynamic uplift.
238 O. Catuneanu / Journal of African Earth Sciences 38 (2004) 225–242
response to lower subduction rates), and occur with a
higher frequency in the middle stage when subductionand dynamic loading are most active. For example,
flexural cycles in the filled Western Canada foreland
system occurred over time scales of less than 1 My
(Catuneanu et al., 1997b), whereas the overfilled stage of
the basin recorded flexural cycles over 1 My in duration
(Catuneanu and Sweet, 1999). The underfilled and
overfilled stages of the Karoo Basin also recorded a
flexural cyclicity of over 1 My in duration (Catuneanuet al., 1998).
This general scenario of evolution of a foreland sys-
tem is supported by the case studies of the Karoo and
Western Canada basins. Simplified versions of this sce-
nario may also occur, when the rates of dynamic sub-
sidence are never high enough to outpace the rates of
flexural uplift. In such cases (e.g., the Witwatersrand
Basin of South Africa; Catuneanu, 2001), the forebulgeregion does not preserve a stratigraphic record, and
sedimentation is restricted to the foredeep and the back-
bulge flexural provinces. Irrespective of the relative rates
of dynamic subsidence, a common theme among all
foreland systems emerges, which is that at least the early
and late stages of evolution are always dominated by
flexural tectonics. For this reason, basal (forebulge)
unconformities always form at the onset of tectonicloading, while the foredeep is underfilled because of the
low sediment supply. As the orogenic wedge rises above
the sea level and sediment supply increases, the foredeep
becomes filled and then overfilled towards the final
phases of foreland basin evolution.
4.2. Higher-frequency foreland cycles
Superimposed on the first-order foreland cycle,
shorter-term changes in the balance between accom-
modation and sedimentation, and implicitly in the
depositional systems that dominate the foreland system,occur as a result of the interplay of flexural tectonics,
dynamic subsidence, eustasy and sediment supply. This
explains the manifestation of second- or lower-order
transgressions and regressions of the interior seaways,
which in effect represent high-frequency changes be-
tween filled (shallow marine) and overfilled (nonmarine)
conditions.
System-wide sedimentation of wedges that show auniform depositional character (e.g., progradational or
retrogradational) is possible when sedimentation con-
sistently outpaces or is outpaced by the rates of base
level rise across the entire foreland system. This de-
scribes a situation where the rates of sedimentation and
base level rise are off-balance, varying within ranges that
do not overlap. Often though, the rates of sedimentation
and base level rise vary within the same range, and thisdelicate balance leads to the formation of reciprocal
(out-of-phase) stratigraphies across flexural hingelines
(Catuneanu et al., 1999a). Two styles of reciprocal
stratigraphies have been defined, for the filled and
overfilled stages of basin evolution, and both refer to
sequences with a cyclicity controlled by flexural tecto-
nism. The filled style of reciprocal stratigraphies requires
base level rise across the entire foreland system (dynamicsubsidence or sea level rise > flexural uplift), and the
coeval backstepping and progradation of the proximal
and distal shorelines of the interior seaway (Fig. 13).
The overfilled style of reciprocal stratigraphies requires
the dominance of flexural tectonism over the long-
wavelength controls on accommodation (dynamic sub-
sidence or sea level rise), which results in a half-system
type of sedimentation (Figs. 14 and 16).The recognition of reciprocal stratigraphies requires
good time control, and can be used to reconstruct the
history of thrusting and off-loading in the adjacent fold-
thrust belt. Each stratigraphic sequence showing a
reciprocal pattern across the flexural hingeline corre-
sponds to a flexural cycle of orogenic loading and
unloading, and the periodicity of such cycles may be
both > and <1 My, as shown by modeling and casestudies. Reciprocal stratigraphies can also be used to
map the geographic location of flexural hingelines, as
well as to monitor their migration through time. Based
1
3
2
O. Catuneanu / Journal of African Earth Sciences 38 (2004) 225–242 239
on such stratigraphic criteria, flexural hingelines oftenseem to be controlled by underlying basement struc-
tures, which explains the mismatch that may be ob-
served between the expected (theoretical) wavelengths
and the real extent of flexural provinces in the field (Fig.
5).
13
2 undeformed lithospheric profile
Fig. 19. Flexural response of a visco-elastic lithosphere that relaxes
stress (modified from Beaumont et al., 1988, 1993). The diagram shows
static loading in the fold-thrust belt, which leads to foredeep subsi-
dence and forebulge uplift. At the same time, the foredeep becomes
deeper and narrower with time (time steps 1–3) in response to the
visco-elastic relaxation of the lithosphere.
4.3. Migration of foreland systems through time
Foreland systems prograde or retrograde through
time in response to the redistribution of orogenic loads(Beaumont et al., 1993; Crampton and Allen, 1995;
DeCelles and Giles, 1996). As a general trend, foreland
systems prograde (shift towards the craton) during early
stages of evolution, and tend to retrograde during their
late stages. The progradation of a foreland system is a
direct response to the progradation of the orogenic load
as thrusting proceeds (Fig. 18). The rates of prograda-
tion depend on the dynamics of the orogenic belt, andthe actual distance of progradation ranges from less
than 100 km (Catuneanu, 2001) to 200 km or more (see
Catuneanu et al., 2000, for a discussion on rates and
amounts of progradation in the case of the Western
Canada foreland system). The retrogradation of a
foreland system may be attributed to piggyback
thrusting accompanied by a retrogradation of the center
of weight within the orogenic belt during orogenicloading, or to the retrogradation of the orogenic load
via the erosion of the orogenic front during times of
orogenic unloading (Catuneanu et al., 1998). The ret-
rogradation of the center of weight in the fold-thrust
belt may also be caused by the redistribution of load in
relation to stages of extension or transtension, as re-
corded in the final phase of evolution of the Canadian
cordillera (Price, 1994; Catuneanu et al., 2000).Independent of tectonic loads, the retrogradation of
flexural hingelines may also be caused by the visco-
elastic relaxation of the lithosphere through time (Fig.
19; Beaumont et al., 1988, 1993). As indicated by
modeling, a lithosphere that is subject to supracrustal
loading tends to relax stress, leading to the deepening
and narrowing of the foredeep with time. This type of
visco-elastic relaxation operates over time scales that are
Distance
Deflection
FOREBULGE
BACK-BULGE
VV
A BFOREDEEP
earliest flexural profilelatest flexural profile
OrogenicLoad
Fig. 18. Progradation through time of a foreland system in response to
the progradation of orogenic load (modified from Beaumont et al.,
1993; Crampton and Allen, 1995; DeCelles and Giles, 1996). ‘‘V’’ re-
fers to the rate of progradation. Vertical scale is exaggerated for
clarity.
larger relative to the cyclicity of flexural loading–
unloading cycles (Beaumont et al., 1988, 1993).
An example of a migrating foredeep basin is offered
by the case study of the Western Canada foreland sys-
tem (Fig. 20). The location of flexural hingelines at
different time steps in the evolution of the basin was
mapped based on stratigraphic criteria (Catuneanuet al., 2000). The tectonic regime of dextral transgression
is known from independent structural studies in the
cordilleran belt (e.g., Price, 1994). The foredeep pro-
graded to the east during the Campanian–Maastrichtian
interval in response to the progradation of orogenic
load, and retrograded in the Paleocene as a result of the
tectonic load redistribution that accompanied the tran-
sition from compression to extension in the fold-thrustbelt. Visco-elastic relaxation of the lithosphere may have
also contributed to the observed retrogradation pattern.
At the same time, the tectonic regime of dextral trans-
gression led to an oblique direction of thrusting, which
resulted in the northward migration (along strike) of the
foredeep basin. It is also important to note that fore-
deeps tend to have a limited lateral extent along strike,
as flexural hingelines curve around the locus of maxi-mum tectonic loading at any given time (Fig. 20).
As a matter of first-order trends, it is conceivable that
the progradation of a foreland system tends to be most
rapid during its early stages of evolution, when the mass
of the tectonic load is still low and hence the rates of
thrusting are highest. In contrast, the rates of visco-
elastic relaxation are expected to increase with time in
response to repeated stress fluctuations (gradual weak-ening of the lithosphere) and increased sedimentary load
in the basin. These opposite trends are illustrated in Fig.
21, and explain why most foreland systems are similar in
terms of migration patterns, recording initial progra-
dation and final retrogradation during their evolution.
The first-order progradation of the forebulge (stage 1
in Fig. 21) results in the formation of the forebulge
(basal) unconformity, commonly found at the base of
Onset of subductionand tectonic loading
End of subductionand tectonic loading}
Foreland cycle
Rat
es
Time
progradation oforogenic front
visco-elasticrelaxation
(1) (2){ {Fig. 21. First-order trends in the evolution of retroarc foreland sys-
tems. The rate of orogenic front progradation decreases through time
as the mass of the orogen increases. The visco-elastic relaxation of the
lithosphere increases through time, leading to an accelerated narrow-
ing and deepening of the foredeep. As the balance between these two
opposite trends changes through time, the foreland cycle may be
subdivided into (1) a first-order stage of forebulge progradation, when
the forebulge (basal) unconformity forms; and (2) a first-order stage of
forebulge retrogradation, when the foredeep accumulates increasingly
thicker sequences with time.
MONTANA
ECM
C
LC
EEM
Calgary
Saskatoon
SASK
AT
CH
EW
AN
AL
BE
RT
A
UTAH
IDAHO
0 km 300
foredeep position during the Early Campanian
indicating hingeline and foredeep migration
orogenic belt flexural hingeline
CANADAU.S.A.
LEM
LM
EP
Edmonton
MC-L
C
EC-MC
LC
-LM
P
Fig. 20. Foredeep position during consecutive time slices of the Early
Campanian–Early Paleocene interval in the evolution of the Western
Canada foreland system (modified from Catuneanu et al., 2000). The
arrows in the orogenic belt indicate the main direction of load shift
during the Early Campanian–Middle Campanian (EC–MC), Middle
Campanian–Late Campanian (MC–LC), Late Campanian–Late Ma-
astrichtian (LC–LM), and Paleocene (P) intervals. This stage was
dominated by dextral transgression in the Canadian cordillera, which
resulted in the northward migration of the foredeep. Trends of pro-
gradation and retrogradation of the flexural hingeline may also be
observed along dip.
240 O. Catuneanu / Journal of African Earth Sciences 38 (2004) 225–242
foredeep deposits (Fig. 6). The age-equivalent foredeep
strata of this hiatus are generally overthrusted and
incorporated within the structures of the orogenic belt.
In the case of the Karoo Basin, the early shift of the
forebulge during the 330–300 My interval was likely in
excess of 450 km, as no sediments of this age are pre-served in the Cape Fold Belt or in the undeformed basin
to the north. As documented by stratigraphic studies of
the preserved Karoo Basin, the northward migration of
the forebulge continued after the Late Carboniferous
with another ca. 200 km during the Permian (Catuneanu
et al., 1998). A decrease in progradation rates may
therefore be inferred, from at least 15 km/My during the
Carboniferous, to maximum 5 km/My towards the endof the Permian.
The first-order retrogradation of the forebulge (stage
2 in Fig. 21) in response to the gradual increase in the
rates of visco-elastic relaxation is accompanied by a
corresponding increase in the rates of creation of accom-
modation in the foredeep. As a result, the younger
foredeep sequences tend to be thicker, as observed in the
Western Canada Basin. In this case study, the thicken-ing upward trend of foredeep sequences records a
change of one order of magnitude, from the 100–101 m
thick sequences of the filled stage (Catuneanu et al.,
1997b) to the 101–102 m thick sequences of the overfilled
stage (Catuneanu and Sweet, 1999).
4.4. Phanerozoic vs. Precambrian foreland systems
Foreland systems irrespective of age are the productof the same allogenic mechanisms, which control their
formation and evolution through time. Similarities are
therefore expected in terms of shapes, stages of evolu-
tion, and stratigraphic architectures. Two important
differences are however related to the age of the under-
lying lithosphere relative to the age of the basin, and the
dynamics of plate tectonic processes at the time of basin
formation.One of the oldest and best preserved Precambrian
retroarc foreland systems in the world is the Late Ar-
chean (ca. 3.0–2.8 Gy) Witwatersrand Basin of South
Africa. The sedimentary fill of the foredeep shows the
same wedge-shape geometry as all Phanerozoic coun-
terparts, as well as the typical transition from underfilled
(West Rand Group) to overfilled (Central Rand Group)
stages (Catuneanu, 2001). The wavelength of the flexural
O. Catuneanu / Journal of African Earth Sciences 38 (2004) 225–242 241
profile is however much shorter (about half) comparedto Phanerozoic basins like the Karoo or the Western
Canada, suggesting a different lithospheric rheology at
the time of formation. This rheological contrast is due to
the fact that the Witwatersrand Basin developed on a
young (newly cratonized), hotter and hence less rigid,
underlying lithosphere (Catuneanu, 2001). This rela-
tively short wavelength may therefore be a distinctive
feature of all Precambrian foreland systems, as theyformed on young visco-elastic plates with a finite
relaxation time.
The second major factor that influenced the evolution
of Precambrian foreland systems is the erratic character
of plate tectonic dynamics in its early stages (Eriksson
and Catuneanu, 2004). The interaction between plate
tectonics and mantle plumes was a unique first-order
control on Precambrian continental drift, and causedthe rates of extension or compression to vary signifi-
cantly within a wide range, generally much larger than
the one known for the Phanerozoic. The rates of sub-
duction are proportional to the magnitude of dynamic
loading, and hence are directly relevant to the evolution
of any retroarc foreland system. These rates are gener-
ally considered to have been higher in the Precambrian,
due to the higher geothermal gradients and the morerapid convection of sublithospheric mantle cells. This
would tend to lead to increased rates of dynamic sub-
sidence in the Precambrian retroarc foreland settings.
Mantle plumes however also interfered with the
dynamics of the early Earth, and often led to the ‘‘sus-
pension’’ of continents over the uprising mantle material
(Eriksson and Catuneanu, 2004). This suspension would
temporarily freeze plate tectonic processes, leading to adecrease in the rates of subduction and hence a cessation
of dynamic loading. This seems to be the case with the
Witwatersrand foreland system, where the forebulge
region was probably never lowered below the base level,
being represented in the rock record by an unconformity
(Catuneanu, 2001).
Acknowledgements
Financial support during the completion of this work
was provided by the University of Alberta and NSERCCanada. My knowledge and ideas about the geology
and evolution of foreland systems has benefited con-
siderably from discussions and collaboration with An-
drew Miall, Chris Beaumont, Glen Stockmal, Jerry
Mitrovica, Art Sweet, and many others. Feedback from
Andrew Miall, Art Sweet and Pat Eriksson helped to
improve earlier versions of this manuscript. I also wish
to thank Pat Eriksson for his exceptional editorial sup-port.
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